The present invention relates to an apparatus and a method for detecting a short circuit to the battery voltage when driving piezoelectric elements.
Piezoelectric elements can be used as actuators because, as is known, they possess the property of contracting or expanding as a function of a voltage applied thereto or occurring therein.
The practical implementation of actuators using piezoelectric elements proves to be advantageous in particular if the actuator in question must perform rapid and/or frequent movements.
The use of piezoelectric elements as actuators proves to be advantageous, inter alia, in fuel injection nozzles for internal combustion engines. Reference is made, for example, to EP 0 371 469 B1 and to EP 0 379 182 B1 regarding the usability of piezoelectric elements in fuel injection nozzles.
Piezoelectric elements are capacitative elements which, as already partially alluded to above, contract and expand in accordance with the particular charge state or the voltage occurring therein or applied thereto. In the example of a fuel injection nozzle, expansion and contraction of piezoelectric elements is used to control valves that manipulate the linear strokes of injection needles. The use of piezoelectric elements with double acting, double seat valves to control corresponding injection needles in a fuel injection system is shown in German patent applications DE 197 42 073 A1 and DE 197 29 844 A1, which are incorporated herein in their entirety.
Fuel injection systems using piezoelectric elements are characterized by the fact that, to a first approximation, piezoelectric elements exhibit a proportional relationship between applied voltage and the linear expansion. In a fuel injection nozzle, for example, implemented as a double acting, double seat valve to control the linear stroke of a needle for fuel injection into a cylinder of an internal combustion engine, the amount of fuel injected into a corresponding cylinder is a function of the time the valve is open, and in the case of the use of a piezoelectric element, the activation voltage applied to the piezoelectric element.
FIG. 1 is a schematic representation of a fuel injection system using a piezoelectric element 2010 as an actuator. Referring to FIG. 1, the piezoelectric element 2010 is electrically energized to expand and contract in response to a given activation voltage. The piezoelectric element 2010 is coupled to a piston 2015. In the expanded state, the piezoelectric element 2010 causes the piston 2015 to protrude into a hydraulic adapter 2020 which contains a hydraulic fluid, for example fuel. As a result of the piezoelectric element""s expansion, a double acting control valve 2025 is hydraulically pushed away from hydraulic adapter 2020 and the valve plug 2035 is extended away from a first closed position 2040. The combination of double acting control valve 2025 and hollow bore 2050 is often referred to as double acting, double seat valve for the reason that when piezoelectric element 2010 is in an unexcited state, the double acting control valve 2025 rests in its first closed position 2040. On the other hand, when the piezoelectric element 2010 is fully extended, it rests in its second closed position 2030. The later position of valve plug 2035 is schematically represented with ghost lines in FIG. 1.
The fuel injection system comprises an injection needle 2070 allowing for injection of fuel from a pressurized fuel supply line 2060 into the cylinder (not shown). When the piezoelectric element 2010 is unexcited or when it is fully extended, the double acting control valve 2025 rests respectively in its first closed position 2040 or in its second closed position 2030. In either case, the hydraulic rail pressure maintains injection needle 2070 at a closed position. Thus, the fuel mixture does not enter into the cylinder (not shown). Conversely, when the piezoelectric element 2010 is excited such that double acting control valve 2025 is in the so-called mid-position with respect to the hollow bore 2050, then there is a pressure drop in the pressurized fuel supply line 2060. This pressure drop results in a pressure differential in the pressurized fuel supply line 2060 between the top and the bottom of the injection needle 2070 so that the injection needle 2070 is lifted allowing for fuel injection into the cylinder (not shown).
It is therefore an object of the present invention to develop the apparatus as defined in the preamble of claim 1 and the method as defined in the preamble of claim 6 to reliably detect a short circuit to the battery voltage within, or at the terminals of, one or more of the piezoelectric elements.
This object is achieved, according to the present invention, by providing a device and a method as described herein.
These provide for:
an element selection switch for selecting the piezoelectric element for operation, an operation selection switch for selecting a charging or discharging circuit to be connected to the piezoelectric element, and a control unit that activates the element selection switch for a predetermined time interval before activating the operation selection switch, the control unit further generating an error signal if a current measurement is greater than a predetermined maximum during the predetermined time interval; and for
closing the element selection switch of the piezoelectric element, monitoring a current measurement for a predetermined time interval before closing a switch to select a charging or discharging circuit to connect to the piezoelectric element, and generating an error signal if the current measurement during the predetermined time interval is greater than a predetermined maximum.
In the embodiment described herein, an actuator driving circuit drives groups of piezoelectric elements which are connected in parallel with one another. By operation of switches, each piezoelectric element is isolated and operated separately from the others during a fuel injection cycle.
A short circuit to the battery voltage has many undesirable effects which prevent proper driving of the piezoelectric elements. For example, a short at the positive terminal of a piezoelectric element will prevent it, and other piezoelectric elements connected in parallel with it, from discharging totally during a discharging cycle. A short to the battery voltage at the negative terminal of an unselected piezoelectric element could cause the piezoelectric element to be improperly charged by forming a closed circuit through the branch of a separate selected actuator connected in parallel. A short to the battery voltage at the negative terminal could also prevent a piezoelectric element from charging properly during its intended charging cycle because the polarity of the charging voltage would be opposite from the polarity of the short circuit voltage. The presence of the battery voltage directly at the piezoelectric elements defeats the ability of the piezoelectric element driver circuitry to regulate administration of voltage in an even stepped fashion. A possible consequence of a short from the piezoelectric element to the battery voltage is the unintentional injection of fuel; a situation which is extremely undesirable. Furthermore, a short to the battery voltage at the negative terminal would cause an undesired high current when the corresponding selection switch is closed. This could destroy the current sensor (shunt resistor).
The piezoelectric element driver circuitry, according to the present invention, includes a number of switches to carry out the charging and discharging cycles for the piezoelectric elements in a fuel injection system. Some switches are used to select which piezoelectric element will be operated at a given time. Other switches control whether a charging or discharging circuit will be connected to a selected piezoelectric element. The operation of these switches is controlled by preprogrammed activation IC and control unit microprocessors.
According to the present invention, in order to detect a short circuit to the battery voltage the piezoelectric element selection and operation switches are activated in a distinct sequence while currents in the piezoelectric element driver circuitry are monitored.
First, the piezoelectric element selection switches are activated to select a particular piezoelectric element to be operated. At this time, the operation selection switches have not been activated to connect a charging or discharging circuit to the piezoelectric element. Thus, under normal conditions, no electrical current should flow through any of the piezoelectric elements, or through a piezoelectric shunt resistor connected in series with the piezoelectric elements. If a short circuit to the battery voltage is present at a piezoelectric element, an abnormal current will flow from the short circuit, through any intervening piezoelectric elements, through the piezoelectric shunt resistor, and finally to ground. Thus by monitoring for such an abnormal current for a predetermined time interval after selecting a piezoelectric element, a short circuit to battery voltage can be detected using an apparatus and/or method according to the present invention. If a current greater than a predetermined maximum is detected an error signal is generated, and cycling of the piezoelectric elements 10, 20, 30, 40, 50, and 60 is halted. If no currents above the predetermined maximum are detected, then the driving circuit will proceed normally to connect the appropriate circuit for charging or discharging the selected piezoelectric element.